It seems that the reason they are preferring the KJV is the "antiquated" English which is harder for non-English speaking people to understand. The third article also states

"The Castro regime does NOT favor the King James translation, which is in English, but rather theReina ValeraSpanish translation, which dates back to the early 17th century and is therefore nearly as antiquated as the King James."

But it's still an older Spanish dialect...and I'm not sure if they speak pure Castillian (major dialect) Spanish in Cuba anymore. Anyway, I'm just glad they're allowing Bibles into Cuba. It's where my family is from and someday I want to go back there. Maybe on a missions trip...that'd be the best!